1. Field of the Invention
This invention relates to multiplexing and demultiplexing image data.
2. Description of the Prior Art
In known image transmission or storage apparatus, representations of the luminance and chrominance components of an image are transmitted or stored in digital form. An example of such an apparatus is a component digital video tape recorder (DVTR) in which representations of the luminance and chrominance components of successive fields of a video signal are recorded by one or more rotary magnetic heads as successive slant recording tracks on a magnetic tape medium. During normal replay, one or more rotary magnetic replay heads follow the recorded slant tracks and read the image data recorded thereon.
A problem occurs during shuttle (high speed) replay of a magnetic tape recorded on a DVTR. During shuttle replay it is necessary to reproduce the recorded luminance and chrominance image information with sufficient quality that an operator can locate an approximate desired position on the tape. The longitudinal tape speed in shuttle replay is greater than that used during recording, which has the effect that a rotary magnetic replay head does not follow a single recorded slant track along the track's entire active length, but instead crosses a number of recorded slant tracks as it traverses the tape. This means that if a rotary magnetic replay head first crosses a recorded slant track part of the way through the image data recorded on the track, it may not be possible to determine which portion of the image corresponds to the image data read by the replay head from that track. Also, because only part of the image data on each recorded slant track is read, it is possible that the luminance and chrominance information used to reconstruct a particular portion of a single video field may have originated in two or more different fields. This may cause subjective distortion of the reconstructed images.
This problem can be particularly severe in a DVTR employing a data compression system to reduce the quantity of data which has to be recorded by the DVTR. One proposed method of image data compression is to use a data quantiser followed by an entropy encoder to effect compression of the image data, with the size of the resultant compressed image data depending on the parameters used and the information content of the particular image. In these systems a compressed representation of the image is recorded on the magnetic tape medium. Because the image data has been subjected to compression resulting in compressed data whose length depends on the image's information content, image data recorded at corresponding positions on two tracks may relate to entirely different portions of the representation. This means that it is particularly difficult to determine which portion of the representation corresponds to the image data read by a rotary replay head as the head crosses from one recorded slant track to another during shuttle replay.